Mining method involving sliding block of overburden on gel lubricant

ABSTRACT

A method for exposing sedimentary mineral deposits, such as coal, by shifting massive, unbroken blocks of overburden on a Bingham plastic lubricant film, such as a clay-water paste, wherein, the overburden blocks are severed on vertical planes, by trenching and/or fracturing and severed along a basal plane, above the mineral deposit, by hydraulic fracturing or undercutting, and a lubricant is injected along said basal plane; the overburden blocks are shifted into a space, created in front of said blocks, by excavation or by previously shifted blocks, from which space the mineral deposit has been removed and in which space a ramp of fill, lubricated with Bingham plastic material, has been prepared over which the overburden blocks will move, by impelling the overburden blocks down slope, by gravity, or by a hydrostatic load of mud dammed up behind said overburden blocks; and, following the shifting of blocks in a first row, the operation is continued by removal of the mineral deposit exposed behind the shifted row of blocks, severing and lubricating a next row of blocks, preparing new earthern ramps and shifting the newly formed blocks.

FIPYEHI 0? 3199179349 Umted States Patent [191 [111 3,917,349

Cleary, Jr. Nov. 4, 1975 MINING METHOD INVOLVING SLIDING such as coal, by shifting massive, unbroken blocks of BLOCK OF OVERBURDEN ON GEL overburden on a Bingham plastic lubricant film, such LUBRICANT as a clay-water paste, wherein, the overburden blocks are severed on vertical planes, by trenching and/or fracturing and severed along a basal plane, above the 225 2 1 a mineral deposit, by hydraulic fracturing or undercutting, and a lubricant is injected along said basal plane;

[22] Filed: Apr. 16, 1974 the overburden blocks are shifted into a space, created in 'front of said blocks, by excavation or by previ- [21] Appl' 461301 ously shifted blocks, from which space the mineral deposit has been removed and in which space a ramp of {76] Inventor: James Mansfield Cleary, 11., 92

[52] U5. Cl. 299/13; 299/19; 299/18 fi lubricated with Bingham plast m r a has been [51] Int. CI. E21C 37/12; E21C 41/00 prep e which the overburden blocks will move, [58] Field of Search 299/11, 13, 18, 19; 61/35 y mp lling the overburden lo k wn l pe, by gravity, or by a hydrostatic load of mud dammed up [56] References Cited behind said overburden blocks; and, following the UNITED STATES PATENTS shifting of blocks in a first row, the operation is continued by removal of the mineral deposit exposed behind the shifted row of blocks, severing and lubricating a next row of blocks, preparing new earthern ramps and shifting the newly formed blocks.

3,274,783 9/ l966 Handy 3,762,771 10/1973 Livingston 3,792,906 2/1974 Kuck 299/18 Primary ExaminerErnest R. Purser Attorney, Agent, or FirmCharles F. Steininger 56 Claims, 1 Drawing Figure [57] ABSTRACT A method for exposing sedimentary mineral deposits,

US. Patent Nov. 4, 1975 Sheet 1 of7 3,917,349

U.S. Patent Nov. 4, 1975 Sheet 3 of7 3,917,349

Fig.3

Sheet 4 of 7 3,917,349

US. Patent Nov. 4, 1975 US. Patent Nov. 4, 1975 Sheet5 of7 3,917,349

US. Patent Nov. 4, 1975 Sheet 6 of 7 3,917,349

U.S. Patent Nov. 4, 1975 Sheet 7 of? 3,917,349

MINING METHOD INVOLVING SLIDING BLOCK OF OVERBURDEN ON GEL LUBRICANT BACKGROUND OF THE INVENTION to a technique for the mining of near surface deposits of minerals, such as, coal, rock, etc., particularly deposits having a gently dipping and reasonably flat top surface. I

In present day techniques for mining near surface deposits of minerals, such as coal and the like, the body of earth or overburden located on top of the mineral deposit is first removed. This removal of the overburden may be accomplished in a number of ways, but irrespective of the manner of removal, the basic operation consists of stripping the overburden to expose the mineral deposit and depositing the stripped overburden in a mound on the surface of the earth adjacent the area being mined. Hence, this type of operation has been termed strip mining. The removal of the overburden, in strip mining operations, is generally performed by scraper-type, earth-moving machines or bucket-type cranes. As previously indicated, the removed overburden is deposited in mounds adjacent the area being mined. If the mineral deposit is a relatively soft material, it can be removed by scraper-type, earth-moving machines or bucket-type cranes. However, in most cases, the mineral deposit is of sufficient hardness that it must first be broken up, as by blasting techniques or the like, and then loaded into trucks or the like and removed.

Such strip mining techniques are relatively inexpen-' sive as compared with sinking a shaft from thesurface to the mineral deposit, tunneling out the mineral deposit and transporting it to the surface through the tunnel system and out the shaft. However, strip mining is still expensive because of the necessity of removing substantial volumes of overburden to. get to the subsurface mineral deposit. More importantly, such strip mining operations always result in complete destruction of the countryside where the operation is being carried out. In many cases, a substantial trench is left in the earths surface and mounds of the overburden are left scattered over the countryside. While some political jurisdictions require the operator to leave the countryside in essentially the same condition as it was prior to the commencement of the strip mining operation, these requirements are rather toothless laws and the end result is generally ;not aesthetically pleasing. Obviously, when. the overburden is removed, all trees and vegetation on the surface must be removed and destroyed. Consequently, at the termination of the strip mining operation, there is a cavernous trench and a moonscape of spoil piles. The typical reclamation procedure is to level the area to some degree and spread some seed and fertilizer. However,.the latter is generally ineffective. In order for vegetation to be re-es'tablished, many, manyyears of growth are required and such growth is never the equivalent of the original vegetation. ,The top soil, containing nutrients capable of supporting vegetation has been mixed with or completely covered by subsurface clay, shale, rock and debris from the mineral deposit. Consequently, such debris is generally incapable of supporting any vegetation and, at best, only the poorest and sparsest of vegetation. In addition, strip mining operations normally in- 2 clude the removal and dumping of toxic materials, such as sulfides, etc., on the surface. Such toxic materials are usually in the waste or debris separated from mineral deposits, such as shale from coal, which are, therefore, brought to the surface and dumped along with the overburden materials. Even in those cases where refilling and'leveling is required, these objectionable materials lie on the surface of the earth for substantial periods of time, prior to being used as fill. Sulfuric acid, a product i of sulfide weathering, and silt are washed into bodies of water and across the countryside by rain and snow. Thus, vegetation in large areas surrounding the strip mining operation is often killed and bodies of water are polluted to the point where they are incapable of supporting life and are unfit for human consumption.

It is therefore highly desirable to provide a mining technique for the removal of near surface mineral deposits and the like which is essentially less expensive than present day strip mining operations. It is also highly desirable to provide such a mining technique which can be carried out with a minimum of disturbance of the surface of the earth, particularly vegetation. It is also highly desirable to provide a technique for the mining of near surface deposits of minerals and the like which minimizes the deposition of deleterious materials on or near the surface of the earth.

SUMMARY OF THE INVENTION In accordance with the present invention, near surface mineral deposits and the like are mined by first removing the overburden to expose the mineral deposit in a relatively small area and provide a cavity having a near vertical cut on one side, removing the mineral deposit thus exposed, thereafter severing a block of overburden, adjacent the cavity, vertically and along a generally horizontal plane, preferably corresponding to the bedding plane between the overburden and the mineral deposit,,injecting a layer of lubricant between the severed block of overburden and the top of the mineral deposit, sliding the block of overburden along the lubricant layer into the previously mentioned cavity, removing the mineral deposit in the area exposed by the removal of the block of overburden and thereafter repeating the steps of severing, moving the blocks of overburden and removing the mineral deposit and moving the subsequently severed blocks of overburden into the cavities left by the movement of previous blocks of overburden and the removal of mineral deposits thereunder.

The technique of the present invention has numerous advantages over the'prior art method previously discussed. First of all, except for a relatively small excavation, which is removed by conventional means in the preliminary site preparation, massive blocks of overburden are removed from the mineral deposit as unitarymasses rather than by conventional scraping and digging and, thus, the expense of such conventional scraping and digging is considerably reduced. Secondly, the movement of massive blocks of overburden isalso sufficiently inexpensive to permit the mining of thinner mineral deposits that could not be economically mined by conventional strip mining operations. Of prime importance, however, are the advantages of minimal environmental damage. Again, except for the starting excavation and the drilling of shot holes and injection wells and forming narrow trenches (hereinafter described), the surface topography and vegetation is minimally disturbed. Furthermore, since only a relatively small starting excavation is necessary, the materials removed from the starting excavation can be segregated and placed in separate mounds for subsequent filling. Then, when the mining operation is completed and the final cavity is to be filled, rock or shale may be deposited first, then clay and then top soil. Thus, the fill is perfectly capable of supporting vegetation and undesirable debris is deposited a sufficient depth below the surface that it will not be washed across the countryside and into bodies of water after filling has been completed. In addition, since massive blocks of overburden are moved, there is no disturbance of the natural layering of the overburden and the exposure of deleterious materials therein. It is also possible to utilize native clays and water, which can be found at the site of the mining operation, to form the lubricant for moving the blocks and thus contribute to the economy of the operation. It is also possible to carry out the mining operation without bringing waste, which may contain deleterious materials, such as sulfurous shale from coal, to the surface and such waste materials can be used in the movement of the blocks by forming inclined ramps or bearing surfaces for the movement of the blocks of overburden. Thus, the dangers and disadvantages of pollution of the countryside and streams are avoided.

More specifically, the preliminary site is prepared by scraping or digging an initial, generally-rectangular starting excavation. Adjacent the middle of one of the long sides of the excavation, an access ramp into the excavation is also formed. This same side of the cavity or excavation forms the front or high walls of two blocks of overburden, which are subsequently re moved, on either side of the access ramp. The mineral deposit is then removed from the starting excavation and any slag, debris or other waste separated from the mineral deposit is graded to form an inclined ramp or sliding surface extending from the high wall over the width of the excavated cavity.

Thereafter, a block is prepared for movement by outlining the block with a series of vertical trenches, fractures or shot holes, in which explosives are discharged, to vertically sever the overburden block down to the mineral deposit. The block is also severed adjacent to or along the bedding plane between the mineral deposit and the overburden block by undercutting, fracturing, drilling and shooting or other known means, such as those utilized in the coal mining industry to produce an undercut or kerf at the bottom of a coal deposit. At least one injection well is drilled in the block of overburden, which can also be used in the conventional manner to create a horizontal fracture at or above the interface between the mineral deposit and the overburden. After the block has been severed, a clay paste (or other suitable lubricant) is formed by mixing clay and water and is injected into the well to form a layer of such lubricant under the block of overburden.

Displacement of the block of overburden is accomplished by sliding the block along the layer of lubricant, down the ramp formed of debris and into the starting excavation. A portion of the lubricant, previously used, can be recovered and used again to displace subsequent blocks of overburden. Movement of the block of overburden may be accomplished in several ways. In cases where the lubricant is injected along a bedding plane, the mine should preferably be laid out so the dip of the bedding is in the direction of block motion. This will, of course, supply, by gravity, some or all of the energy necessary to move the block. In some cases, the

driving energy will be obtained from a head of mud dammed up behind the block. Within limits, the head of mud can be pumped up to maintain block movement. Such movement can be aided by appropriately placed explosive charges. Such explosive charges may also be utilized in helping to guide the block in the proper direction, to the extent that it tends to move in the wrong direction. Where mud is used behind the block to initiate and maintain the movement of the block, it may be retained in the channel behind the block by building up a dam along the side with heavy lubricant or loose fill or debris. Movement and control of overburden blocks may also be affected to some degree by heavy traction or hydraulic jacking equipment, but the large scale of most operations will tend to make such efforts relatively ineffective.

After a block has been removed, removal of the mineral deposit can be carried out by conventional means, as by breaking up the deposit with explosive charges and then loading and hauling the mineral to the surface up the access ramp. Any waste or debris separated from the mineral deposit during the mining of the mineral is then carefully graded to provide an inclined ramp or bearing surface from the high wall or front of the next block of overburden to be removed, sloping downwardly in the cavity left by the removal of the first block of overburden and the mineral deposit thereunder. The same procedure of severing another block and moving such block is then performed to deposit the second block in the cavity left by the first. In moving subsequent blocks and preparing subsequent blocks for movement, a partition equal in width to the access ramp can be left between two blocks being prepared for movement, and this partition can then be graded, as necessary, to extend the access ramp. Further, clay paste or lubricant utilized for moving one block can be scraped from the top surface of the mineral deposit for reuse on the graded bearing surface for subsequent block movements.

The above-mentioned objectives and advantages and the operation of the present technique will be apparent from the following detailed description when read in conjunction with the drawings wherein:

FIG. 1 shows a simplified, perspective view of a starting space or excavation for carrying out the method of the present invention;

FIGS. 2A and 2B show the operational sequence for the generation of a narrow trench with explosives;

FIG. 3 shows an elevational view, partially in section, of the injection well preparation for production of a hydraulic fracture severing a block of overburden along a bedding plane between the mineral deposit and the overburden block;

FIG. 4 is a plan view showing a possible sequence of moving overburden blocks together with means for undercutting the blocks at the interface between the'mineral deposit and the overburden block;

FIG. 5 is an elevational view, in cross section, showing the nature of the layer of lubricant between an overburden block and the bearing-surface beneath it;

FIG. 6 is an elevational view, in cross section, showing an overburden block which has been moved and one which is about to be moved;

FIG. 7 is an elevational view, in cross section, showing means of guiding overburden blocks during movement;

FIGS. 8A and 8B show a plan view and an elevational view, in cross section, respectively, of an overburden block prior to movement; and 1 FIGS. 9A, 9B and 9C show a simplified plan view of the sequence of operations utilized in moving overburden blocks by the hydrostatic drive method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preliminary Site Preparation In accordance with FIG. 1, the mining technique of the present invention is begun by first forming a starting excavation 10 by conventional surface mining procedures. Starting excavation 10 has one side forming a vertical cut 12 which becomes the front faces of the first overburden blocks 14 to be moved. The starting excavation will preferably be made along an outcrop of the mineral deposit or where the overburden is espe cially thin. In some cases, an existing stripping operation could be used as the starting excavation 10, in which case the vertical cut 12 would be the long high wall of the stripping operation. In the embodiment of the mining technique depicted in FIG. 1, a trench 16, wide enough to use as a haulage road, is excavated back from vertical cut 12. The far end of the trench forms a ramp 13 to the original ground level 21. As the operation advances by the movement of succeeding rows of blocks, the trench is extended and road access to the pit formed behind the blocks is maintained.

Severing of Block of Overburden and Placement of Lubricant The block of overburden 14 is vertically severed by outlining the same with a series of shot holes 18', which are loaded with an appropriate explosive which is dis charged to form vertical fractures along the lines of the shot holes, as will be hereinafter described in greater detail. Injection wells 20 are also drilled from the surface of the blocks of overburden to the top of the mineral deposit. Injection wells 20 are used for hydraulic fracturing at the interface between'the bottom of the block of overburden and the top of the mineral deposit and for injecting lubricant, all as hereinafter described in greater detail.

As indicated previously, overburden blocks 14 are prepared for displacement by severing the same along vertical planes extending from the earths surface to the top of the mineral or coal deposit 22 and along a base plane between overburden blocks 14 and coal deposit 22, preferably along the bedding plane between the two. The vertical severance can be performed in numerous ways.

Separation of the overburden blocks, along their vertical faces, from the surrounding overburden may involve several widely different processes depending on the needs of the particular situation. The wide trench 16 shown in FIG. 1 would be cut by conventional excavation techniques.

The most economical block severing technique is the propagation of a hydraulic fracture along a line of mud filled shot holes 18 by the use of explosives. Very little energy is required to propagate a fracture in rock once the fracture has been initiated. This fact, pointed out in US. Pat. No. 703,302, derives from the low tensile strength of rock, and the tensile stress concentration at the extending edge of a pressure induced fracture. Consequently, the required fractures can be produced with a relatively wide hole spacing and low explosive loading. A preferred method would be to use a low velocity 6 explosive placed in the bottom of the mud filled holes. The fractures will then be extended by the impulsively driven mud as well as by the gasses from the explosive. Some advantage may be gained in controlling the initial fracture orientation by grooving opposite sides of the bore hole, as taught in US. Pat. No. 342,366. The grooves would lie in the plane of the desired fracture.

The use of a pressure induced fracture for overburden block separation is applicable to the rear face and the side faces when movement of the block will produce a separation of the two rock surfaces. This is true for the cases shown in FIGS. 1 and 4. In some cases, however, a narrow trench rather than a fracture will be required to permit block movement parallel to the vertically-disposed surfaces. These trenches can be generated, at least in part, by conventional trenching equipment of the chain saw type, a bucket wheel or a ladder trencher having toothed buckets driven by a continuous chain. However, the depth capability of available equipment is limited. A narrow trench may also be produced by the use of explosives in a line of shot holes.

A preferred method for generating a narrow trench by the use of explosives is illustrated in FIGS. 2A and 2B. Starting at the high wall 12, holes 18 are drilled and shot in sequence along the line of the intended trench 19. Each hole is filled with mud or water, then loaded with a centralized linear charge of explosive 25. Each hole is loaded and shot after drilling so that the progress of the operation is like that shown in FIGS. 2A and 2B (from right to left). For the first few shots, the material is blown cleanly from the trench. In later shots, a portion of the broken material is expelled from the trench, in the jet of mud and gas, but the main part is propelled into the void left by the previous shots. This mixture of broken rock and mud 23 will not require removal if its state of compaction and consequent shear strength can be kept at reasonable levels. The primary application for narrow trenches will be for the hydrostatic drive method of block displacement taken up in the later discussion. As will be later understood, shear strengths of several hundred pounds per square foot will not be excessive because of the very large driving forces available from the hydrostatic drive technique. Nevertheless, it is desirable to keep the shear strength of the material filling the trench as low as possible. To'this end, water should be poured into the fill after the trenching operation has been com pleted to minimize its later compaction.

A method for severing the overburden block 14 along its base plane of separation above the coal, and ultimately placing the clay lubricant under the block of overburden 14, is to employ hydraulic fracturing, a process widely used in oil production for stimulating flow from a well. When a bore hole is hydraulically fractured, fluid is pumped into the bore hole under pressure sufficient to fracture the surrounding rock. The fracture can then be extended by continued injection. The fluid pressure required to extend the fracture is only a little greater than the compressive stress in the rock normal to the plane of the fracture, for horizontal fractures about I psi. per foot of depth. In a deep well, hydraulic fracture orientation will generally be controlled by the native rock stresses. The fracture tends to orient normal to the smallest compressive rock stress. Vertical hydraulic fractures are therefore common because the horizontal stress is often substantially less than the vertical stress. In the case of immediate interest, the compressive rock stresses are small. The orientation of the initial fracture will be controlled by notching the bore hole, as shown in FIG. 3, so that the vertical tensile stress concentration at the tip of the notch 24 will cause a horizontal fracture to be initiated. The well bore notch 24 will be centered in a bedding plane 26 in a stratified shale which is judged to be a plane of easy parting. The fracture therefore continues to extend in this bedding plane 26. FIG. 3 illustrates the preparation of the injection well before hydraulic fracturing. The injection well 2t) will have a casing 28 cemented therein. A drill pipe 30, lowered into well 20, has a centralized nozzle assembly 32 attached to its lower end. A deep notch is cut by a jet of abrasive fluid 34 delivered by the nozzle assembly 32 as the pipe and nozzle assembly 32 are rotated.

The fracture will be initiated and extended with water or a low viscosity drilling mud through a well 20 near the center of the overburden block to be moved. After continuing injection for a time, the fracture will be observed to break out on the outside of the block. The viscosity of the injection fluid is then gradually in creased until a high gel strength clay paste, used as lubricant, can be injected at reasonable pressure. If the high gel strength material were injected at the beginning, injection pressures would be excessive. By first widening the fracture with low viscosity material, the injection pressures can be maintained at reasonable levels.

By continued injection of the clay paste, the fracture is widened until a thick film of lubricant has been deposited under the block Eventually an equilibrium film thickness is reached when the clay paste extrudes from under the block as fast as it is being injected.

A mud pump, such as is used in deep oil well drilling, would be suitable for clay paste injection. Preferably, the valves and manifolding would be modified to provide maximum flow area. A means for delivering positive pressure to the suction will be required. A vibrating hopper close-connected above the suction might be used to feed paste to the pump. Equipment used in brick manufacturing, for mixing and extruding clay, might also be used to feed clay paste to the mud pump.

Another way to establish a lubricated bearing surface under the overburden block 14, as illustrated in FIG. 4, is to mechanically cut a deep slot in or above the coal deposit 22 and inject the clay through shot holes 18 penetrating the overburden along the back of the slot. The slot is preferably cut at an angle to provide a favorably inclined bearing surface.

Mechanical undercutting can best be applied to a long high wall 112 so the undercutting machine 36 can operate continuously. Active or abandoned stripping operations would provide economical starting excavations 10 for this type of operation.

FIG. 4 illustrates a preferred layout of the operation. The undercutting machine 36 advances continuously along the base of the high wall 12. Behind the advancing undercutter, the floor of the cavity is graded for passage of the overburden block 14411. A truck mounted drilling rig 38 works along the top of the high wall l2 drilling the shot holes 18., injecting clay and shooting loose the sections of overburden 14. The blocks M then slide over to the opposite wall, as illustrated by blocks 14k and MI. Clay is injected as close behind the slotting machine 36 as practical to help support the overhang and to prevent spalling from the roof of the slot. As coal is mined from the newly formed cavity, waste or spoil from the operation is used to rough 8 grade the floor of the cavity in preparation for the movement of the next line of blocks 14.

The width of the cavity is maintained wider than the blocks 14 being moved. A roadway 40 is thereby kept open between the new and the old cavity. Successive cavities will have a tendency to become narrower if the blocks 14 fail to move solidly together. Periodic widening of the cavity may therefore be necessary.

Undercutting permits control over the quality and inclination of the bearing surfaces so that a strong gravity drive may be obtained together with consistently low sliding resistance. A typical example is given below in which the following defined symbols are used:

L overburden block length normal to sliding direction, ft.

8 overburden block width in the sliding direction,

H thickness of overburden block, ft.

a angle of inclination of sliding surfaces, degrees W average density of sediments of overburden block, lb./ft.

T lubricant clay gel strength, lb./ft.

Assumed values for the example depicted are as follows:

L 100 ft.

B 50 ft.

H 100 ft.

144 lb/ft T 40 lb/ft The weight of the block is:

W LBHw 72 million pounds, the sliding resistance is:

F BLT 0.2 million pounds, and the gravitational driving force is:

F tanaW 6.3 million pounds. Thus, the driving force is 3l times the calculated sliding resistance. In 75 ft. of travel, the impact with the opposite wall would be about 20 ft/sec. The above example shows that driving forces much greater than expected drag forces can readily be obtained.

The preferred methods for clay film placement are compared as follows:

Hydraulic Fracture Film Placement Advantages l. The clay paste can be injected economically over a wide area. There is no limit to the block size.

Disadvantages 1. It is difficult to predict the quality of the fracture surfaces generated. They are likely to be less fiat than mechanically cut surfaces and will require relatively thick films of relatively high gel strength clay paste with relatively high sliding resistance.

2. The sliding surface must follow a bedding plane so there is no independent control of inclination.

3. Highly variable drag forces are likely in practice.

Clay Film Injected in Mechanically Cut Slot Advantages l. A high quality sliding surface can be generated. Consequently, a relatively thin low gel strength clay film may be used. Corollary advantages of this are that the block will have relatively low sliding resistance and the cost of manufacturing and pumping the clay paste will be low.

2. The angle of the sliding surface can be controlled independently of dip to obtain any gravitational 9 driving force required to move the block.

Disadvantages l. Undercutting capability requires a large piece of equipment.

2. Depth of undercut is limited by the size of undercutting machine. A 50 Ft. undercut, for example, would require a very large machine.

A most significant factor in the movement of an overburden block is the nature of the lubricant utilized. In the discussion which follows, the following defined symbols will be used, in addition to those previously defined:

V velocity, ft/sec z fill height of mud, ft.

[1 clay film thickness, ft.

W weight of overburden block, lb.

w, mud density, lb/ft.

F g gravitational driving force, lb.

F frictional drag force, lb.

P pressure, lb/ft Plane Slider Bearing and Clay Paste Lubricant While other high gel strength materials, such as guar gum gel, may be used as the lubricating film in the present application, a clay paste is the ideal material in nearly all respects. It is very likely that a suitable native clay will be found in the section above the coal. The only processing required is grinding and mixing with water.

The flow characteristics of clay-water mixtures classify them as Bingham plastics. These fluids, like Neutonian fluids, have a linear relation between shear stress and shear rate, but the Bingham plastic is distinguished by having a gel strength. The shear stress must exceed the gel strength for flow to occur.

A very wide'range of flow properties may be obtained with clay-water mixtures and these properties are remarkably stable. A clay-water mixture having a 55 percent water content might typically have a viscosity of 0.3 poise and a gel strength of 3 lb/ft With percent water content, the gel strength will be about 200 lblft A range of gel strengths between about 1 and 300 lb/ft might be applied in the present technique, while a preferred range is about 20 to 100 lb/ft Consider a rectangular block at rest above a parallel flat surface with an interposed thick lubricant film. If a vertical load is suddenly applied to the block, the lubricant film begins to squeeze out. If the lubricant is Neutonian, the surfaces are held out of contact for a short time, during which the coefficient of friction of the bearing is low. Soon, however, the film thins to the point where surface contact supports the entire load and the friction coefficient is high. If the lubricant film is clay paste, it begins squeezing out as the load is ap plied, but an equilibrium film thickness is soon attained whose value depends on the gel strength of the clay paste. A pressure distribution in the clay film supports the load. The following is a generalized expression for equilibrium film thickness.

, where l 11 is the film thickness at the trailing edge of the block, L is the length of the block normal to the direction of block motion, B is the block length in the direction of block motion, Tis gel strength of the clay film and W is 10 the bearing load. K is a function of L/B, Il /I1 and whether or not the block is sliding, I1 is the leading edge film thickness. Table 1 gives values of K for a range of L/B and 11 /11 values. Table 1 gives two sets of K values, one for the static condition, the other for the block moving slowly.

Table I L, h,/h Average K K Film Static Sliding Thickness Block Block 1 1 1h: .33 0 1 2 l.5h .23 .102 l 4 25h: .15 .095 4 l lh .46 0 4 2 l.5h .31 .20 4 4 2.5h .20 .183 x l ihg .5 0 I 2 1.5h .34 .234 x 4 2.5h- .22 72?.

The solutions obtained for the sliding block case are quite involved and inexact. They rely heavily on theory and calculated results given in Sternlicht & Pinkus, Theory of Hydrodynamic Lubrication, McGraw- Hill, 1961. However, equation (1) and the values of K in Table I will serve as a rough guide to the film thickness that can be deposited and maintained for different block dimensions and lubricant gel strength.

The values of K in formula (1) given in Table I show that the equilibrium film thickness for a static block is generally greater than for the moving block. So, if the maximum static film thickness were placed under the block, the film thickness would decline as the block moved forward approaching equilibrium film thickness for the sliding block. Another unsteady state effect will be the tendency of the block to change inclination as it moves. Assuming that a wedge-shaped film thickness distribution is established at the onset, if the center of gravity of the block is located over the center of the bearing area, the favorable film thickness distribution will gradually disappear as the block slides. If the shape of the lower bearing surface increases in slope and is convex upward, the favorable film thickness distribution will be maintained despite the tendency of the block to tilt forward.

If the block sliding velocity were large, viscous shear stresses would add significantly to the shear stresses in the film and film thicknesses would be greater than predicted by equation l Sliding resistance also increases with block velocity. At low sliding velocity the sliding resistance, F, is given approximately by F TBL (2) Certain regions of the lubricant film may be stagnant because the shear stress is less than the gel strength.

The stagnant boundary layers lie within the concaved regions in the rough bearing surfaces and, if Il /I1 2, the stagnant layer is substantially thickened on the upper surface near the leading edge and on the lower surface near the trailing edge. (See Sternlicht & Pinkus, supra). In FIG. 5, the boundaries between the stagnant regions 42 and the shear region 44 are represented by dashed lines. The effective bearing geometry is determined by these boundaries rather than the actual solid boundaries. Those portions of the surface roughness covered by the stagnant boundary layer have no influence on the behavior of the bearing.

If the block 14 with clay paste lubricant film is moved a significant distance, a supply of lubricant must be provided at the leading edge to maintain the film. As indicated by the K values in Table I, to maintain the thickness of the film the slider must have a positive angle of attack, that is, its bottom surface should incline upward in the direction of motion (Il /I1 1). The most straightforward way to accomplish this is to place the load center about 8 percent of the block length behind the center of the bearing area. The film will then assume a favorable thickness distribution and the viscous pumping action will maintain film thickness. If the block is centrally loaded, the film thickness will decrease as the block travels and eventually the drag force would greatly increase.

The following methods could be used in order to maintain a favorable film thickness distribution during the motion of the overburden block:

1. The block can be cut so that both the front and rear faces angle backward, placing the center of mass behind the center of the bearing area. (See FIG. 6.) This method is applicable when the sediments are reasonably well consolidated and when H B.

2. An injection well in the forward half of the bearing area can be used to increase the film thickness in this area as needed.

3. If the block has a favorable angle of attack, as a starting condition, the film thickness will be maintained for a limited travel distance because the block angle changes very slowly with displacement.

4. This favorable angle of attack can be maintained by grading the bearing surface in front of the block at a greater angle than the bearing surface of the block and having the slope increase away from the block. This feature is also shown in FIG. 6.

To convert a massive rectangular block of overburden into a low friction bearing, it is, of course, necessary to deposit a clay film thick enough for the waviness and surface roughness of the bearing surfaces not to contact each other. Without attempting to characterize these surface irregularities, we can say that some film thickness, 12, exists which is sufficient to keep the surfaces out of contact. The required value of 11 will vary greatly and in some cases it will be too large for practicality. However, for each value of 11, we can calculate the value of T required and the resulting resistance to block motion. For example, assume a block of overburden with dimensions L 200 ft, B 50 ft, H 75 ft, 21 required film thickness at the trailing edge h 0.05 ft, a film thickness at the leading edge l1 1 0.2 ft,

W BLHw, or W 108 million pounds, (3)

L/B 4 and /z //z 4, so, from Table I, K 0.22 for a sliding block. Then substituting in (1 the required gel strength is T 49 lb/ft and from (2) the sliding resistance is F 0.5 million lb. If the sliding surfaces are inclined at an angle a, the gravitational component directed down slope, F is P tan a W.

If the slope equals that required for the block to slide without the aid of an external driving force, then F F and from (3) and (2) CUZ.7/9F '1' tano: W In the above example, a would then equal 026. In most real cases, it seems likely that prominences in the sliding surfaces will come in contact and bear a fraction of the overburden load so that the actual friction would normally be a multiple of the value calculated above. But these results indicate that the sliding resistance is potentially very low.

FIG. 6 illustrates a section through an overburden block 14 prepared to slide on a clay film 54. The fill 52 has been graded in the mined out area in front of the block at an angle greater than the dip of the clay filled slot. The graded surface is convex increasing in slope away from the block. Then, as the overburden block 14 overides the clay lubricant, the lubricant film 56 in front of the block becomes thicker and the slope becomes greater during travel. Thus, the clay paste 56, heaped in front of the block, is spread down as a film under the advancing block.

In order to end up flush with the opposite wall and butted close to the previously moved blocks, it is important that the moving block follow the correct path. One method for steering the block is to shape a guiding rib in the undercut slot as it is cut. FIG. 7 suggests another way the block might be guided. A guide rail 62 is installed in the slot at the center of the block 14 and lined up with the desired direction of motion. The guide 62 would have a seriated lower surface to prevent sliding and a sharp rib on its top surface. The rib would bite into the underside of the sliding block 14 and guide it in the proper direction. With either of these two methods, if a steep sliding angle were used, momentum would build up rapidly during the early guided part of travel. There would then be little time for disorientation over the final distance of travel. A substantial impact velocity would also serve to fit the block closely with the opposite wall, despite surface roughness.

Hydrostatic Drive FIGS. 8 and 9 illustrate the site preparation, for a slip mining operation, which makes use of a head of mud behind the block of overburden 14 to drive it forward. Two widely spaced trenches 19 are cut perpendicular to the high wall 12. A line of shot holes 18 joins the two trenches 19. Clay paste is injected through wells 20 located near the center line of the block 14 into a bedding plane above the coal 22. The strip pit area 10 in front of the block 14 is graded to provide a bearing surface 52.

A mud storage pit is prepared a substantial distance back from the high wall 12 on high ground so mud will flow by gravity to the working area as needed. The mud, prepared from water and clay, has low viscosity and gel strength in comparison with the clay paste lubricant. The mud is preferably a free flowing drilling mud-type material, as used in oil well drilling. The primary reason for using drilling mud instead of plain water is that mud, because of its ability to build filter cake over fractures and permeable surfaces, will be much easier to retain in the pit behind the block 14 being displaced. The sealing qualities can be improved by incorporating particulate material in the mud, such as saw dust or chopped rubber tires. In deep well drilling, a wide variety of such materials, termed lost circulation materials, are used in drilling mud. For example, a loose earth fill could be used to block flow in the side trenches 19. Because of filter cake build up, mud loss through a dam of earth fill would be insignificant. Typical properties for a slurry made with 22 percent native clay are as follows:

Density 72 lb/ft Viscosity l cp.

Gel strength 0.l lb/ft These properties might vary widely without influencing the operation.

The lines of shot holes 18 is filled with mud. Shooting the mud filled holes separates the overburden block 14 at the rear face. Mud flowing by gravity from a storage pit fills the fracture at the rear of the block 14. As the block 14 begins to move, the mud continues to flow into the widening trench maintaining the fluid level and hydrostatic head required to keep the block moving. The clay paste or loose fill in the side trenches forms a dam preventing loss of mud from the rear trench.

As an example, let

H=100 ft L 600 ft B 200 ft T 40 lb/ft W, 144 lb/ft and The force applied by the mud column at the rear of the block is F /i L Z w (s) Letting Z H, the maximum load is then 216 million pounds.

I If a thick clay film is deposited at the base of the block producing complete support of the block by film pressure, the sliding resistance is F LBT 4.8 million pounds. Thus, the available driving force is about 45 times the minimum sliding resistance.

The pumping energy requirern ent, if the mud must be returned to the storage pit after the block 14 is moved, is given by BLZ Where Z is taken as the average fill height and letting Z 33 ft,

E 7.84 X 10? ft. lb. or about 4,000 horsepower hours.

With hydrostatic drive, the block will generally move in small increments with a stick-slip motion. As the block moves ahead, the driving head will decline until it falls below the head necessary to keep the block in motion. The head will then build up until the block is set in motion again. It is likely that the center of sliding resistance will always be somewhat off the block center so the block will have a tendency to turn. One method for guiding and straightening the block would be to build the fluid head to a level a little less than that expected to set the block in motion and then to set off an explosive chargeunder the mud at the end of the block that is lagging. Since the block would be on the verge of moving, the momentary pressure wave would move the lagging end of the block ahead. Another guidance technique would be to inject additional clay paste through the injection well at the lagging end of the block during the course of block movement.

In operations involving hydraulic fracture film placement, the dip of the bedding may have a significant effect on block movement. If the dip is significant, the operation will be greatly aided if the mine can be laid out so that blocks will move in the down dip direction, and greatly hindered if the converse is true. In the previous example, a l dip would result in a 3.8 million pound horizontal force component.

While specific techniques and materials have been exemplified herein, it is to be understood that these details are given for illustrative purposes only and many variations and modifications thereof will be apparent to one skilled in the art. Accordingly, the present invention includes all such variations and modifications and the invention is to be limited only by the appended claims.

What is claimed is:

l. A method of moving a massive, substantially unbroken block of earthoverburden to expose a portion of a subsurface formation beneath said overburden; comprising, separating said massive, substantially unbroken block of overburden from the adjacent overburden with which it initially forms a unitary mass, along generallywertical planes and along a generallyhorizontal, base plane adjacent the top of said subsurface formation; disposing a thin layer of a high gel strength lubricantpaste beneath said block of overburden in the plane of separation along said generally-horizontal base plane; and sliding said substantially unbroken block of overburden on said layer of lubricant paste in a direction and for a distance sufficient to expose said portion of said subsurface formation.

2. A method in accordance with claim 1 wherein the base plane generally follows a bedding plane near the top of the subsurface formation.

3. A method in accordance with claim 2 wherein the bedding and base plane dip downward in the proposed direction of the overburden block movement.

4. A method in accordance with claim 1 wherein the subsurface earth formation is a mineral deposit.

5. A method in accordance with claim 1 wherein the mineral deposit is a coal deposit.

6. A method in accordance with claim 1 wherein the block of overburden is separated from the adjacent overburden by trenching so as to deliniate the block at least part way down to the top of the subsurface forma- I 115 tion.

'7. A method in accordance with claim 6 wherein the trenching extends from the surface of the earth to the top of the subsurface formation.

8. A method in accordance with claim 6 wherein the trenching extends from the surface of the earth to a point part way to the top of the subsurface formation and the separation is extended to said top of said subsurface formation by discharging an explosive.

9. A method in accordance with claim 8 wherein the explosive is disposed in at least one hole drilled from the bottom of the trench to the top of the subsurface formation.

It). A method in accordance with claim 6 wherein the trenching extends from the surface of the earth to a point part way to the top of the subsurface formation and the separation is extended to said top of said subsurface formation by hydraulic fracturing.

11. A method in accordance with claim 10 wherein at least one hole is drilled from the bottom of the trench to the top of the subsurface formation and a hydraulic fracturing fluid is pumped into said hole.

12. A method in accordance with claim 1 wherein the block of overburden is separated from the adjacent overburden by cutting along a line along which the separation is to be made to a point at least part way to the base plane.

13. A method in accordance with claim 12 wherein the cutting is performed by a mechanical cutting apparatus.

M. A method in accordance with claim 12 wherein the cutting extends from the surface of the earth to the base plane.

15. A method in accordance with claim 12 wherein the cutting extends from the surface of the earth to a point part way to the base plane and the separation is extended to said top of said base plane by discharging an explosive.

16. A method in accordance with claim 15 wherein the explosive is disposed in at least one hole drilled in the same plane as the cut to the base plane.

l7. A method in accordance with claim l2 wherein the cutting extends from the surface of the earth to a point part way to the base plane and the cut is extended to said base plane by hydraulic fracturing.

18. A method in accordance with claim 17 wherein at least one hole is drilled in the same plane as the cut to the base plane and a hydraulic fracturing fluid is pumped into said hole.

19. A method in accordance with claim 1 wherein the block of overburden is separated from the adjacent overburden by the discharge of an explosive.

20. A method in accordance with claim 19 wherein at least one hole is drilled on the line along which the separation is to be made and the explosive is discharged in said hole.

21. A method in accordance with claim 20 wherein the hole is notched in the direction of the line of separation prior to discharging the explosive.

22. A method in accordance with claim it wherein the block of overburden is separated from the adjacent overburden by hydraulic fracturing.

23. A method in accordance with claim 22 wherein at least one hole is drilled on the line along which the sep aration is to be made and a hydraulic fracturing fluid is pumped into said hole.

24. A method in accordance with claim 23 wherein the hole is notched in the direction of the line of separation prior to pumping of the fracturing fluid.

25. A method in accordance with claim 1 wherein the block of overburden is separated along the base pla e by cutting a slot at least part way along said base plane.

26. A method in accordance with claim 25 wherein the cutting is performed by a mechanical cutting appa- I'iltUS.

27. A method in accordance with claim 25 wherein the cutting extends part way along the base plane and the separation is extended along said base plane by hydraulic fracturing.

28. A method in accordance with claim 25 wherein the slot is out along the base plane at a downward inclination toward the proposed direction of block movement.

29. A method in accordance with claim 1 wherein the block of overburden is separated along the base plane by hydraulic fracturing.

30. A method in accordance with claim 29 wherein at least one hole is drilled through the block of overburden from the surface of the earth to the base plane and a fluid is pumped into said hole with pressure sufficient to produce hydraulic fracturing at the bottom of said hole.

31. A method in accordance with claim 29 wherein the hole is notched in the base plane prior to pumping the fracturing fluid.

32. A method in accordance with claim 29 wherein the lubricant is pumped into the hole after the fracturing fluid.

33. A method in accordance with claim 32 wherein the lubricant is a claywater mixture.

34. A method in accordance with claim 32 wherein the lubricant has the characteristics of a Bingham plastic.

35. A method in accordance with claim 32 wherein the viscosity of the fracturing fluid is 1651. than the viscosity of the lubricant.

36. A method in accordance with claim 35 wherein the viscosity of the fracturing fluid is gradually increased until the viscosity of the lubricant is reached.

37. A method in accordance with claim 36 wherein both the fracturing fluid and the lubricant are claywater mixtures.

38. A method in accordance with claim 29 wherein the base plane coincides with a natural formation bedding plane adjacent the top of the subsurface forma tion.

39. A method in accordance with claim 1 wherein the lubricant is injected into the separation along the base plane.

40. A method in accordance with claim ll wherein the lubricant is a clay-water mixture.

41. A method in accordance with claim 1 wherein the lubricant has the characteristics of a Bingharn plastic.

42. A method in accordance with claim i wherein the block of overburden is slidably moved into an excavation adjacent said block of overburden.

43. A method in accordance with claim wherein the excavation is formed prior to the movement of the first block of overburden.

44. A method in accordance with claim 42 wherein the excavation is formed by the previous movement of a block of overburden and the removal of the subsurface formation beneath said previously moved bloclt of overburden.

45. A method in accordance with claim 42 wherein an inclined ramp is formed from the front of the block of overburden, with respect to the direction of movement, downwardly toward the floor of the excavation.

46. A method in accordance with claim 45 wherein the ramp has an inclination greater than the inclination of the base plane.

47. A method in accordance with claim 45 wherein the inclination of the ramp increases with distance from the overburden block.

48. A method in accordance with claim 45 wherein the ramp is composed of earth fill.

49. A method in accordance with claim 45 wherein lubricant is disposed on top of the ramp adjacent the block of overburden.

50. A method in accordance with claim 1 wherein the block of overburden is moved by introducing a body of 18 liquid behind the block of overburden, with respect to the direction of movement.

51. A method in accordance with claim 50 wherein the'head of liquid is maintained at the level required to complete the movement of the block of overburden.

52. A method in accordance with claim 50 wherein at least one dam is formed along at least one side of the block of overburden to retain the body of liquid behind said block of overburden.

53. A method in accordance with claim 52 wherein the dam is formed from loose fill.

54. A method in accordance with claim 52 wherein the dam is formed from lubricant.

55. A method in accordance with claim 50 wherein the body of liquid is a body of water.

56. A method in accordance with claim 50 wherein the body of liquid is a body of clay-water mixture. 

1. A method of moving a massive, substantially unbroken block of earth overburden to expose a portion of a subsurface formation beneath said overburden; comprising, separating said massive, substantially unbroken block of overburden from the adjacent overburden with which it initially forms a unitary mass, along generally-vertical planes and along a generally-horizontal, base plane adjacent the top of said subsurface formation; disposing a thin layer of a high gel strength lubricant paste beneath said block of overburden in the plane of separation along said generally-horizontal base plane; and sliding said substantially unbroken block of overburden on said layer of lubricant paste in a direction and for a distance sufficient to expose said portion of said subsurface formation.
 2. A method in accordance with claim 1 wherein the base plane generally follows a bedding plane near the top of the subsurface formation.
 3. A method in accordance with claim 2 wherein the bedding and base plane dip downward in the proposed direction of the overburden block movement.
 4. A method in accordance with claim 1 wherein the subsurface earth formation is a mineral deposit.
 5. A method in accordance with claim 1 wherein the mineral deposit is a coal deposit.
 6. A method in accordance with claim 1 wherein the block of overburden is separated from the adjacent overburden by trenching so as to deliniate the block at least part way down to the top of the subsurface formation.
 7. A method in accordance with claim 6 wherein the trenching extends from the surface of the earth to the top of the subsurface formation.
 8. A method in accordance with claim 6 wherein the trenching extends from the surface of the earth to a point part way to the top of the subsurface formation and the separation is extended to said top of said subsurface formation by discharging an explosive.
 9. A method in accordance with claim 8 wherein the explosive is disposed in at least one hole drilled from the bottom of the trench to the top of the subsurface formation.
 10. A method in accordance with claim 6 wherein the trenching extends from the surface of the earth to a point part way to the top of the subsurface formation and the separation is extended to said top of said subsurface formation by hydraulic fracturing.
 11. A method in accordance with claim 10 wherein at least one hole is drilled from the bottom of the trench to the top of the subsurface formation and a hydraulic fracturing fluid is pumped into said hole.
 12. A method in accordance with claim 1 wherein the block of overburden is separated from the adjacent overburden by cutting along a line along which the separation is to be made to a point at least part way to the base plane.
 13. A method in accordance with claim 12 wherein the cutting is performed by a mechanical cutting apparatus.
 14. A method in accordance with claim 12 wherein the cutting extends from the surface of the earth to the base plane.
 15. A method in accordance with claim 12 wherein the cutting extends from the surface of the earth to a point part way to the base plane and the separation is extended to said top of said base plane by discharging an explosive.
 16. A method in accordance with claim 15 wherein the explosive is disposed in at least one hole drilled in the same plane as the cut to the base plane.
 17. A method in accordance with claim 12 wherein the cutting extends from the surface of the earth to a point part way to the base plane and the cut is extended to said base plane by hydraulic fracturing.
 18. A method in accordance with claim 17 wherein at least one hole is drilled in the same plane as the cut to the base plane and a hydraulic fracturing fluid is pumped into said hole.
 19. A method in accordance with claim 1 wherein the block of overburden is separated from the adjacent overburden by the discharge of an explosive.
 20. A method in accordance with claim 19 wherein at least one hole is drilled on the line along which the separation is to be made and the explosive is discharged in said hole.
 21. A method in accordance with claim 20 wherein the hole is notched in the direction of the line of separation prior to discharging the explosive.
 22. A method in accordance with claim 1 wherein the block of overburden is separated from the adjacent overburden by hydraulic fracturing.
 23. A method in accordance with claim 22 wherein at least one hole is drilled on the line along which the separation is to be made and a hydraulic fracturing fluid is pumped into said hole.
 24. A method in accordance with claim 23 wherein the hole is notched in the direction of the line of separation prior to pumping of the fracturing fluid.
 25. A method in accordance with claim 1 wherein the block of overburden is separated along the base plane by cutting a slot at least part way along said base plane.
 26. A method in accordance with claim 25 wherein the cutting is performed by a mechanical cutting apparatus.
 27. A method in accordance with claim 25 wherein the cutting extends part way along the base plane and the separation is extended along said base plane by hydraulic fracturing.
 28. A method in accordance with claim 25 wherein the slot is cut along the base plane at a downward inclination toward the proposed direction of block movement.
 29. A method in accordance with claim 1 wherein the block of overburden is separated along the base plane by hydraulic fracturing.
 30. A method in accordance with claim 29 wherein at least one hole is drilled through the block of overburden from the surface of the earth to the base plane and a fluid is pumped into said hole with pressure sufficient to produce hydraulic fracturing at the bottom of said hole.
 31. A method in accordance with claim 29 wherein the hole is notched in the base plane prior to pumping the fracturing fluid.
 32. A method in accordance with claim 29 wherein the lubricant is pumped into the hole after the fracturing fluid.
 33. A method in accordance with claim 32 wherein the lubricant is a clay-water mixture.
 34. A method in accordance with claim 32 wherein the lubricant has the characteristics of a Bingham plastic.
 35. A method in accordance with claim 32 wherein the viscosity of the fracturing fluid is less than the viscosity of the lubricant.
 36. A method in accordance with claim 35 wherein the viscosity of the fracturing fluid is gradually increased until the viscosity of the lubricant is reached.
 37. A method in accordance with claim 36 wherein both the fracturing fluid and the lubricant are clay-water mixtures.
 38. A method in accordance with claim 29 wherein the base plane coincides with a natural formation bedding plane adjacent the top of the subsurface formation.
 39. A method in accordance with claim 1 wherein the lubricant is injected into the separation along the base plane.
 40. A method in accordance with claim 1 wherein the lubricant is a clay-water mixture.
 41. A method in accordance with claim 1 wherein the lubricant has the characteristics of a Bingham plastic.
 42. A method in accordance with claim 1 wherein the block of overburden is slidably moved into an excavation adjacent said block of overburden.
 43. A method in accordance with claim 42 wherein the excavation is formed prior to the movement of the first block of overburden.
 44. A method in accordance with claim 42 wherein the excavation is formed by the previous movement of a block of overburden and the removal of the subsurface formation beneath said previously moved block of overburden.
 45. A method in accordance with claim 42 wherein an inclined ramp is formed from the front of the block of overburden, with respect to the direction of movement, downwardly toward the floor of the excavation.
 46. A method in accordance with claim 45 wherein the ramp has an inclination greater than the inclination of the base plane.
 47. A method in accordance with claim 45 wherein the inclination of the ramp increases with distance from the overburden block.
 48. A method in accordance with claim 45 wherein the ramp is composed of earth fill.
 49. A method in accordance with claim 45 wherein lubricant is disposed on top of the ramp adjacent the block of overburden.
 50. A method in accordance with claim 1 wherein the block of overburden is moved by introducing a body of liquid behind the block of overburden, with respect to the direction of movement.
 51. A method in accordance with claim 50 wherein the head of liquid is maintained at the level required to complete the movement of the block of overburden.
 52. A method in accordance with claim 50 wherein at least one dam is formed along at least one side of the block of overburden to retain the body of liquid behind said block of overburden.
 53. A method in accordance with claim 52 wherein the dam is formed from loose fill.
 54. A method in accordance with claim 52 wherein the dam is formed from lubricant.
 55. A method in accordance with claim 50 wherein the body of liquid is a body of water.
 56. A method in accordance with claim 50 wherein the body of liquid is a body of clay-water mixture. 